Dry low NOx hydrocarbon combustion apparatus

ABSTRACT

A fuel with or without fuel bound nitrogen (FBN) is burned in a high pressure machine (20 atm) comprising fuel and air compressors, combustor and turbine at an ER of about 2-3 and temperature below the threshold for creating thermal NO x . Prompt and FBN NO x  are avoided due to the rich mixture, having a dearth of O and OH, producing CO and H 2  and little CH. The turbine cools the products to reduce their temperature. The cooled products are mixed with the remaining air and burned at a temperature below the thermal NO 2  threshold temperature at an ER of about 0.6. Commercial stand alone machines can be used for the rich and for the lean combustors wherein air and fuel are supplied to the rich combustor and only air and the cooled combustion products of the rich machine are supplied to the lean combustor.

This application is a continuation of application Ser. No. 07/328,213,filed Mar. 24, 1989, and now abandoned.

This invention relates to hydrocarbon fuel burning processes and, moreparticularly, to such processes which include methodology for reducingNO_(x) combustion products.

Hydrocarbon fuel burning processes are widely used in stationarypower-generating gas-turbine systems. Combustion by-products whichpollute the atmosphere are required to be minimized as pan of a growingconcern about the quality of the earth's atmosphere. Therefore,combustors for stationary power-generating gas-turbine systems arerequired to produce low levels of nitric oxides (NO, NO₂, N₂ O, etc.,collectively referred to as NO_(x)) and of CO. Such emissions lead toacid rain and other environmental problems. The NO_(x) can result fromreactions with atmospheric nitrogen, such reactions being referred to as"thermal" and "prompt" NO_(x), or with fuel-bound nitrogen (FBN).According to well-supported combustion theory, NO_(x) produced by the"thermal" mechanism is due to atmospheric nitrogen being fixed by theradicals responsible for flame initiation and propagation, as shown bythe following:

    N.sub.2 +O=NO+N

    N+O.sub.2 =NO+O

    N+OH=NO+H

with the net reaction rate approximately given by ##EQU1## in SystemInternational (S.I.) units. Because of the large activation energy inthe exponential term, the formation rate of NO_(x) is not significantbelow about 2780° F., accounting for the descriptor "thermal".

The concentration of certain radical species is also important,particularly at low (on the order of atmospheric) pressures. Theradicals can exist in superequilibrium concentrations as discussed in anarticle by S. M. Correa et al., entitled "Prediction and Measurement ofa Non-equilibrium Turbulent Diffusion Flame," Twentieth (International)Symposium on Combustion, The Combustion Institute, pp. 337-343, 1984,and augment the thermal NO_(x) mechanism. Since radical consumptionreactions speed up at the relatively high pressures in power-generatingsystems, the degree of superequilibrium and the resultant excessradicals are reduced. For a further discussion on the formation ofthermal NO_(x) see the following articles: M. C. Drake et al.,"Superequilibrium and Thermal Nitric Oxide Formation in TurbulentDiffusion Flames", Comb. Flame, 69, pp. 347-365, 1987; "Nitric OxideFormation from Thermal and Fuel-Bound Nitrogen Sources in a TurbulentNon-Premixed Syngas Flame," Twentieth Symposium (Int.) on Combustion,The Combustion Institute, Pittsburgh, Pa., 1983-1990, 1984 and S. M.Correa, "NO_(x) Formation in Lean Premixed Methane Flames", EngineeringSystems Laboratory, 89CRD001, January 1989.

The preponderance of thermal NO_(x) in conventional (fuel and air notpremixed) combustors, due to the high temperatures in the turbulentmixing interfaces, has led to water or steam injection for NO_(x)control. In this approach, the injected water or stem absorbs heat,reduces the peak temperatures (to below the NO_(x) -forming threshold)and so reduces NO_(x) emission levels. The lower temperatures have theundesirable side effect of quenching CO consumption reactions and so theCO levels increase and combustor life and efficiency are reduced. Thusthe water or steam injection technique is not ideal.

Prompt NO_(x) is so named because it is formed very rapidly (inhydrocarbon flames) when atmospheric nitrogen is fixed by alkylradicals, e.g., CH, CH₂, CH₃. The latter occur in the hydrocarboncombustion kinetic chain. The nitrogen is fixed as cyanide (HCN, CN)species which lead to NH_(i) species and are eventually oxidized toNO_(x) by oxygen-containing radicals. The mechanism does not require thehigh temperatures of the thermal mechanism and so prompt NO_(x) is notamenable to control by water or steam injection. FBN NO_(x) is verysimilar in that the fuel-bound nitrogen species are extracted as NH_(i)species which are oxidized to NO_(x). FBN occur for example, in coal,and also in so-called "dirty" gas derived from coal. However, promptNO_(x) is not as much a problem as FBN. In typical applications, FBNNO_(x) can be on the order of 500 ppm or more, while (conventional)combustors with non-FBN fuel have 100-300 ppm thermal NO_(x) and 10-30ppm prompt NO_(x). It would be desirable to burn dirty (FBN) fuel with<100 ppm NO_(x) and clean fuel with <10 ppm NO_(x).

Powerplant constraints dictate that the stability, turn-down ratio (i.e.power changes corresponding to power demand reductions) and efficiencybe similar to those of current equipment. NO_(x) control techniqueswithout water or steam injection are referred to as "dry" combustion.Two dry low-NO_(x) combustion techniques have been suggested (i)rich-lean staged combustion (originally intended for thermal and FBNNO_(x) control but not successful for the reasons discussed below) and(ii) lean premixed combustion (intended for thermal NO_(x) control).

In rich-lean staged combustion, the combustor is divided into a firstzone which is rich (equivalence ratio Φ≅1.3-1.8; note that Φ=1 forstoichiometric conditions, Φ>1 being rich and Φ<1 being lean) and asecond zone which is lean. Because of the off-stoichiometric conditions,temperatures in each zone are too low for NO_(x), (e.g. less than 2780°F.) to form via the "thermal" mechanism.

However in prior art staged systems, the mixing of air with the effluxof the rich zone occurs at finite rates and cannot prevent the formationof hot near-stoichiometric eddies. The attendant high temperatures leadto the copious production of thermal NO_(x), which is triggered attemperatures above about 2780° F. This has been the experience both inthe laboratory and in mainframe (100 MW class) gas-turbine equipment.However, rich combustors are suitable for fuels with a significantfuel-bound nitrogen content because the amount of oxygen available toproduce FBN NO_(x) is limited.

Lean premixed combustors, which are useful if the fuel does not containnitrogen, are fueled by a lean (prevaporized, if liquid fuel) premixedfuel-air stream at Φ≅O.7. The ensuing temperatures are uniformly too low(e.g., less than 2780° F.) to activate the thermal NO_(x) mechanism.Detailed chemical kinetic studies of two such combustors by the presentinventor have lead to the discovery that most of the NO_(x) is producedby the "prompt" NO_(x) mechanism described above (recall that FBN is notpresent). This forms a lower limit to the minimum NO_(x) obtainable incurrent hydrocarbon-fueled combustors. Advanced combustors underdevelopment by the assignee of the present invention have reached anapparent 30-40 ppm NO_(x) barrier (using clean natural gas whichminimizes total NO_(x) production). This barrier can be crossed onlywith an increase in CO and an unacceptable loss of flame stability. Such(lean) combustors also produce unacceptably high levels of NO_(x) fromFBN species in the fuel if FBN species are present. Thus each of theprior art systems has advantages and disadvantages.

According to the present invention, the efflux of a rich combustor iscooled to prevent ignition during mixing to a lean condition. Ignitionand flame stabilization occur only after the lean mixture has beenestablished. According to one embodiment, a portion of the air is burnedunder rich conditions (e.g., overall equivalence ratio, Φ=2.5-3.0) in apreburner to produce a partially combusted stream which contains CO andH₂, referred sometimes as syngas, and very little CH₄ (the originalfuel), CO₂ and H₂ O. The hot gas stream is then cooled by way of exampleby expansion through a turbine or passage through a heat exchanger. Thecooled gas stream is then mixed with the remaining portion of the airstream, without ignition. The lean stream (e.g., Φ=0.5-0.6) is thenburned.

The production of NO_(x) is minimized due to the relatively cooltemperatures in the rich and lean burning cycles, which temperatures arebelow the established level for the production of thermal NO_(x). PromptNO_(x) is also minimized since CH in the lean cycles tends to benegligible. FBN NO_(x) is minimized because the rich combustor runs withtoo little oxygen for production of NO_(x).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a combustion cycle in accordance withone embodiment of the present invention; and

FIG. 2 is a schematic diagram of a combustion cycle in accordance with asecond embodiment of the present invention.

In the schematic diagram of FIG. 1, a representative combustion systemin accordance with one embodiment of the present invention isillustrated. In the system shown a main combustion machine 10 comprisesa compressor 12, a gas mixer 14 and a primary combustor 16 whosecombustion products drive turbine 18. A system comprising compressor 12,mixer 14, combustor 16 and turbine 18 are commercially availablemainframe machines, which for example, may be a General Electric CompanyMS7000 series machine for driving a 100 megawatt class electricalgenerator. A second combustor 20 is coupled to receive 100% of the fuelat in inlet thereof which fuel preferably is methane, coal or coalderived gas or a liquid hydrocarbon fuel. The output of combustor 20 isapplied to a gas cooling stage 22 which may comprise a turbine orexpansion nozzles to cool the gas produced by the combustor 20. Theinlet of combustor 20 receives x% of air from the compressor 12. Theremaining portion of the air 100-x% is applied to the mixer 14 of themainframe machine 10.

Combustor 20 may be similar in construction as the combustor in acommercially available gas turbine generator known as a General ElectricCompany model LM500. The LM500 however, includes a fuel compressor andair compressor for compressing the fuel and air supply to the combustor20. In the embodiment of the machine 10 however, the air compressor isnot included as the air is compressed via compressor 12 and the fuel iscompressed and supplied to the input of combustor 20. The cooling stage22 may include a turbine as available in the model LM500 gas generator.However, a turbine is not essential for the gas cooling stage asindicated above.

100% of the fuel is applied to the combustor 20 which burns that fuel ina rich combustion mixture with a relatively low mount of air suppliedvia the compressor 12. For example, the amount of air supplied tocombustor 20 may be 10% of the air supplied to the mixer 14 fromcompressor 12. The combustion products are applied to the cooling stage22 while at a relatively hot temperature. The temperature is below about2780° F. at which thermal NO_(x) is generated. Because of the richcombustion, little oxygen is available for the combustion process incombustor 20 and the temperature thereof does not exceed the thresholdtemperature at which thermal NO_(x) is generated. The relatively richcharacteristics of the burning process generates little 0, OH and otheroxidizing radicals in the burning process minimizing prompt NO_(x).Also, the rich combustion process favors reforming chemistry, i.e. tendsto avoid the generation of CH gas products; instead produces gasproducts comprising primarily CO and H₂. The CO and H₂ mixture iscommonly referred to as syngas or synthetic gas. The FBN species, ifpresent, are converted to N₂ (molecular nitrogen).

The combustion efficiency of combustor 20 is believed to be generallyabout 75% and therefore about 25% of the fuel in the syngas productsremains unburned. Combustor 20 because it is relatively rich operates atan equivalence ratio (ER) of approximately 2.5-3. Of course, theequivalence ratio will vary between the head end and the exit of thecombustor 20. The exit ER is in the range indicated, the head end beinglower, within the rich stability limit. The combustor 20 is illustrativeof a more complex system in which a staged combustor may be providedwith more fuel added to the products of a rich primary zone having a Φof approximately 2. The added fuel promotes "reforming" chemistry.Because the temperature is below the threshold value for the generationof thermal NO_(x), such thermal NO_(x) is substantially negligible atthe output of combustor 20. The combustor process maximizes CO and H₂and fuel conversion.

The cooling stage 22 may be either a turbine or a heat exchanger to coolthe hot syngas produced by the combustor 20 and deliver power or heat asmy be required in a given implementation. The output of such a turbineor heat exchanger is such to cool the syngas to a temperature belowignition temperatures before delivery to the mixer 14 in the system 10.This step is critical.

In accordance with the principles of the present invention, the syngasproduced by the rich combustor 20 has negligible total NO_(x) because ofthe low temperature and the lack of oxidizing species. FBN species areconvened to N₂. However, in the transition to the mixer 14 it isimportant that the syngas be reduced to a temperature sufficiently lowthat the temperatures in the process of turbulent mixing in mixer 14remain below the threshold for the generation of thermal NO_(x). Withoutthe cooling produced by stage 22 the hot syngas produced by thecombustor 20 when mixed with air in the mixer 14 could lead to ignitionand flame in mixer 14 and to copious thermal NO_(x). Because there islittle CH component in the syngas product of the combustor 20, there islittle prompt NO_(x) in the system 10.

It should be understood that the combustors 16 and 20 include morecomplex combustion systems including primary burners (head ends) anddownstream addition of air in the case of combustor 16, per conventionalpractice, and downstream addition of fuel in the case of combustor 20.Assuming a preburner is included in the combustor 20, then the coolingstage 22 may be provided .Iadd.by .Iaddend.a pressure reducing nozzlewhich will increase the usual approximately 4% pressure drop availablefor mixing. Air needed to premix to lean main-combustor conditions isadmitted via jets within such a nozzle (not shown). With the use of anozzle, integration may be accomplished because the cooling andpremixing can both occur within the nozzle. In this case the mixer 14would be combined in such a nozzle with the mixing occurring in thenozzle. Otherwise the mixer 14 mixes the cooled syngas which is at atemperature below the 2780° F. threshold temperature for the generationof thermal NO_(x), and mixes that air gas at compressor temperature, forexample, 600° F.

The mixing of the syngas with most of the air stream produces a leanpremixed stream having an equivalence ratio Φ of approximately 0.5 atthe head end of the combustor 16 and about 0.3 at the exit. The mixingprocess of mixer 14 or nozzles (not shown) is at a sufficiently lowtemperature so that a flame and thermal NO_(x) cannot be formed duringdilution.

Relatively negligible mounts of hydrocarbons are available at the mixer14 since only air from the compressor 12 is added at the mixer 14 to thesyngas produced by the combustor 20. Therefore, very little promptNO_(x) is generated in combustor 16. The particular operating points forthe fuel and air mixers and pressures and temperatures can be selectedby analysis and experimental variations of the components for a givenimplementation. In particular, the stoichiometries of the combustors 16and 20 are optimized for producing maximum power at the turbine 18. Notshown is an electrical generator or other utilization means coupled tothe turbine 18 and driven thereby.

Because the generation of hydrocarbons and FBN NO_(x) are minimized inthe syngas output or the cooling stage 22 and because the generation ofthe thermal NO_(x) is minimized by maintaining the temperatures belowthe threshold, the fuel supplied to combustor 20 may comprise coal gas,liquid fuels and other types of fuels with relatively high fuel boundnitrogen. Employing the process as discussed above in connection withFIG. 1, the fact that the fuels used in the combustor 20 are rich innitrogen will not affect the resulting products in the syngas at theoutput of the cooling stage 22. Nitrogen in FBN species will beconverted to N₂.

By way of example, combustor 20 may be supplied with approximately 0.5lbs. per second of methane (CH₄) accompanied with 2.5 lbs. per second ofair. The combustor 20 as mentioned above has an overall equivalenceratio of about 3. The syngas output of the cooling state 22 comprisesapproximately a flow rate of 1 lb. per second of carbon monoxide plushydrogen (CO+H₂) the rest being mostly N₂ (nitrogen). This is combinedwith about 15 lbs. per second of air which is applied to the mixer 14.Air for providing dilution and cooling is provided to the combustor 16at approximately 7.5 lbs. per second to provide a downstream exitequivalence ratio Φ of approximately 0.3. This process yields anapproximate NO_(x) level of 5 ppm NO_(x). It should be understood thatthe combustors 20 and 16 are supplied fuel and air at various inputs atthe head end and downstream inputs in accordance with conventionalcombustors. Combustor 16 uses air for downstream inputs while combustor20 uses fuel for downstream inputs.

In FIG. 2, a second embodiment employing two stand alone commerciallyavailable combustion machines are employed for implementing the presentinvention. A compressor 200 compresses all of the hydrocarbon fuel suchas methane to a pressure of about 20 atmospheres and applies thecompressed fuel to combined combustor mixer 202. The compressor 204supplies a portion x% of the total air required overall. Compressor 204supplies the compressed air to the combustor 202. Combustor 202 consistsof a head end operated near the rich limit with downstream addition ofmore fuel to achieve the required stoichiometry. By way of example, xmay be 10% of the air required overall. Combustor 202 burns the fuel airmixture and applies the burned combustion products to a turbine 206. Thepurpose of the turbine 206 is similar to the cooling stage 22 of FIG. 1which provides cooling of the hot combustion gases to produce a cooledcarbon monoxide (CO) and hydrogen (H₂) syngas. The cooled syngas isapplied to the input of mixer 208. The remaining air required is appliedto compressor 210. For example, where 10% of the air is applied tocompressor 204, 90% of the air required to burn overall is applied tocompressor 210. Compressor 210 provides a pressure of about 10atmospheres to the air supplied to the mixer 208. Mixer 208 mixes theair from compressor 210 with the cooled syngas from turbine 206. Themixed cooled gas product is applied to combustor 212 whose hot gasproducts are exhausted to a turbine 214 which drives a generator (notshown).

In one calculation example to verify the process, a 0.5 lbs per secondof methane is supplied as the fuel to compressor 200. This is applied atatmospheric pressure at room temperature (60° F.). To this is added 0.3%NH₃ (ammonia). The ammonia represents fuel bound nitrogen in a coalderived gaseous fuel. The efficiency of fuel compressor 200 is assumedapproximately 0.9. The output of compressor 200 has a temperature ofabout 677° F.

Compressor 204 compresses 2.86 lbs. per second of air supplied atatmospheric pressure at room temperature. The output of compressor 204is at approximately 842° F. with the outputs of both compressors 200 and204 at 20 atmospheres. Combustor 202 mixes the fuel and air and burnsthe combination with Φ at about 2.0 at the head end and 3.0 at the exitport. The output of the rich combustor 202 has a temperature of about2520° F. It is calculated that the products from the combustor 202 haveless than 1 ppm NO_(x) which value increases as the equivalence ratiodecreases. It is also estimated that there are about 750 ppm NH_(i),HCN. The gas products from combustor 202 are applied to turbine 206which runs at about a 2 to 1 pressure ratio which serves to cool thecombustor gas products, producing a cooled syngas on line 207.

Combustor 202 burns a rich fuel air mixture to which more fuel is armedin the downstream region of the combustor. This leads to reduction ofthe initial products by the fuel added downstream. The process isreferred to as "reforming" chemistry such that the products of thesyngas on line 207 are primarily CO and H₂ rather than fuel andcombustion products. The NO_(x) emissions are low on line 207 due to therelatively low temperatures and lack of oxidizing radical species suchas O and OH in combustor 202. This has been verified by laboratoryexperiments and kinetic studies. The output pressure of the turbine 206on line 207 is at about 10.5 atmospheres and at a temperature of about2136° F. If the equivalence ratio in the head end of combustor 202 ismade too high, the flame can become unstable in the combustor. There mayalso be excessive soot because the combination of gas, fuel and air istoo rich. Further, there can be excessive production of NO_(x) as the Φis lowered. For this reason, it is preferred that the head end Φ ofcombustor 202 be in the range of 2 to 2.5, with more fuel addeddownstream.

Compressor 210 receives the remaining air. This air is applied tocompressor 210 at a rate of 25.74 lbs. per second, in this example, atroom temperature and atmospheric pressure. Compressor 210 operates at anefficiency of 0.9. The output pressure of compressor 210 is 10atmospheres at a temperature of about 600° F. This air is mixed in mixer208 with the cooled syngas and applied to lean combustor 212. The leancombustor 212 has a head end Φ of about 0.6 and an exit Φ of about 0.3.Combustion products at the exit of combustor 212 are at about 1860° F.,and exhibit approximately 58 ppm NO_(x) and less than 1 ppm CO. Recallthat the fuel contained FBN (0.3%). Turbine 214 operates with an assumedefficiency of 0.9 and has an output temperature of about 1005° F. at oneatmosphere. The 58 ppm NO_(x) and less than 1 ppm CO products producedby the combustor 212 is considered excellent in view of the combustionof dirty fuel containing 3% ammonia applied to the compressor 200.Normally, such dirty fuel will produce hundreds of ppm of NO_(x). Ofcourse, different ratios of fuel, air and dirty fuel contaminants suchas FBN will produce different values of temperature at the differentstages. The 10% air applied to the compressor 204 and 90% air applied tocompressor 210 is believed optimum for one implementation. Turbine 214is then employed to operate an electric generator or other utilizationmeans.

Turbine 206 causes expansion of the combustor output gases and reducesthe temperature of the syngas to a level where mixing can beaccomplished in mixer 208 without premature ignition. The turbine 206exit pressure is larger than the operating pressure of the combustor 212by about 5% (10.5 atm vs 10 atm) to facilitate mixing of the syngas fromline 207 and the air from compressor 210 to an overall lean condition.The figures given above with respect to the proportions of fuel to air,efficiencies of the compressors and turbines and approximatetemperatures are based on calculations of the various operating points,emissions and overall thermal efficiency. The various assumptions areincluded in these calculations as indicated.

The total fuel and air flow rate are consistent with combustor cans incurrent mainframe power generation machines. Calculation of the cycleefficiency of the embodiment of FIG. 2 shows the cycle efficiency of30.7% to be comparable to a base machine comprising compressor 210,mixer 208 and combustor 212 with the same level accuracy in thecalculation, that is a cycle efficiency of 30.5%. Slight improvement inthe cycle efficiency is due in part to the straight forward improvementof the Brayton cycle with the designated pressure ratios, since thecombustor 202 runs at 20 atmospheres pressure as compared to the 10atmosphere pressure of combustor 212.

What is claimed is:
 1. Gas turbine apparatus for dry low NO_(x)combustion comprising:a first gas turbine set including a firstcompressor means for compressing incoming air to about 20 atmospheres, afirst combustor having fuel injector means and being coupled to saidfirst compressor means, said first combustor having an exit equivalenceratio of about 2.0 to 3.0 so as to produce hot combustion productscomprising substantially CO and H₂ and negligible NO_(x), and firstturbine means coupled to the output of said first combustor for coolingthe hot combustion products below a temperature at which ignition andthermal NO_(x) occur; and a second gas turbine set including a secondcompressor means for compressing incoming air to about 10 atmospheres, amixer coupled to said second compressor means for receiving air fromsaid second compressor means, said mixer also coupled to the output ofsaid first turbine means, the output pressure of said first turbinemeans being about 5% higher than the output of said second compressor.Iadd.means.Iaddend., so that rapid and homogeneous mixing in the mixeris assured, a second combustor having fuel injector means coupled to theoutput of said mixer, said second combustor having an exit equivalenceratio of about 0.2 to 0.3, so that production of NO_(x) and CO isminimized, and second turbine means coupled to the output of said secondcombustor.
 2. The gas turbine apparatus of claim 1, wherein the quantityof air supplied by said second compressor means is approximately tentimes the quantity of air supplied by said first compressor means. 3.The gas turbine apparatus of claim 2 wherein the temperature of theoutput of said first compressor means is approximately 840° F. and thetemperature of the output of said second compressor means isapproximately 600° F.
 4. The apparatus of claim 3 wherein thetemperature of the combustion products at the exit of the first turbinemeans is approximately .[.1860.]. .Iadd.2136.Iaddend.° F. at a pressureof 10.5 atmospheres and the temperature of the combustion products atthe exit of the second turbine means is approximately 1005° F. at apressure of 1 atmosphere.
 5. A gas turbine apparatus for dry low NO_(x)combustion comprising:a compressor for compressing incoming air; a firstcombustor connected to said compressor for receiving a portion of thecompressed air from the compressor and having fuel injection means forreceiving fuel, said first combustor having an exit equivalence ratio ofabout 2.0 to 3.0 so as to produce hot combustion products comprisingsubstantially CO and H₂ .Iadd.and .Iaddend.negligible NO_(x) ; coolingmeans connected to said first combustor for cooling the hot combustionproducts below a temperature at which ignition and thermal NO_(x) occur;a mixer having inlet means and outlet means, said inlet means beingconnected to said cooling means for receiving the cooled combustionproducts and to said compressor for receiving the remaining portion ofthe compressed air; a second combustor connected to said outlet means ofsaid mixer for receiving the output of said mixer, said second combustorhaving an exit equivalence ratio of about 0.2 to 0.3 so that productionof NO_(x) and CO is minimized; and turbine means connected to saidsecond combustor for being driven by the output of said secondcombustor. .Iadd.
 6. A gas turbine apparatus for dry low NO_(x)combustion comprising:a compressor for producing first and secondportions of combustion air; a first combustor receiving said firstportion of combustion air and a fuel and in which combustion productsare produced; cooling means connected to said first combustor forcooling said combustion products; a second combustor receiving saidcooled combustion products and said second portion of combustion air,said second combustor having an exit equivalence ratio sufficiently lowso that very little NO_(x) is generated in a lean combustion process;and a turbine driven by the products of combustion from said secondcombustor. .Iaddend. .Iadd.7. The gas turbine apparatus for dry lowNO_(x) combustion of claim 6 wherein said second combustor includes amixer in which said cooled combustion products and said second portionof combustion air are mixed. .Iaddend. .Iadd.8. A gas turbine apparatusfor dry low NO_(x) combustion comprising: a first compressor forproducing a first portion of combustion air; a second compressor forproducing a second portion of combustion air; a first combustorreceiving said first portion of combustion air and a fuel and in whichcombustion products are produced; cooling means connected to said firstcombustor for cooling said combustion products; a second combustorreceiving said cooled combustion products and said second portion ofcombustion air, said second combustor having an exit equivalence ratiosufficiently low so that very little NO_(x) is generated in a leancombustion process; and a turbine driven by the products of combustionfrom said second combustor. .Iaddend. .Iadd.9. The gas turbine apparatusfor dry low NO_(x) combustion of claim 8 wherein said second combustorincludes a mixer in which said cooled combustion products and saidsecond portion of combustion air are mixed. .Iaddend. .Iadd.10. A methodof generating power comprising the steps of:burning a fuel with a firstportion of combustion air in a first combustor to produce combustionproducts; cooling said combustion products; burning said cooledcombustion products with a second portion of combustion air in a secondcombustor at an equivalence ratio sufficiently low so that very littleNO_(x) is generated in a lean combustion process; and driving a turbinewith the products of combustion of said second combustor. .Iaddend.